Lens system for telescopes



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LENS Np V RADII THICKNESSES I 1.617 36.6 R1 :+6526 '1| 2.6 :1I 1.572 57.4 R2 =+2146NZ 8.0 R5 =162.8 51 47.0 III 1.517 64.5 R4"+74.6O `13= 3.3 R5 $2 N 1.525 548- R@ 16.75 '1,4 2.3 R7 I 1' 30.72 S5 1.3

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WILLY SCHADE INVENTOR ATT'Y AG'T Patented May 4, 1948 LENS SYSTEM FOR TELESCOPES Willy Schade, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application February 14, 1945, serial No. 577,809

'1 claims. (ci. ss-sv) This invention relates to sighting telescopes.

An object of the invention is to provide a sighting telescope with a clear sharp image over an extended field of view and of more economical construction than the sighting devices disclosed in copending application, Serial No. 511,062, by Altman, filed November 20, 1943, and which has become Patent No. 2,430,549, dated November 11, 1947. The Altman invention gives an image quality far superior to anything previously known and the present invention carries these advantages 'into another class of telescopes. The present invention has the added advantage of greatly improved spherochromatism and oblique spherical aberration.

An incidental object of the invention is to provide a form of sighting device which is readily adaptable for use with a scanning prism at some distance in front of the object.

Anobject of one of the particular embodiments described inthis specicatlon was to provide an improved telescopic system to fit into a previously existing mount with no increase in dimensions and with a minimum of mechanical change. The invention is applicable however, to various telescopes and gives highly improved image quality whether space is at a premium or not.

Sighting telescopes consist of an objective system for forming animage, substantially in its l principal focal plane, of the object being sighted and a viewing system for viewing the image. Even -in high quality systems, the viewing system may be nothing more than a compound eyepiece, or it may have one or more relay systems with field lenses interposed near the image plane or planes. Prisms, reticle, beam splitter, filters, and other accessories may also be interposed in known manner.

Prior -to the present invention and that described in the above-mentioned copending Altman application, telescopes have been made with critically sharp definition near the axis but with rapidly deteriorating definition in zones further from the axis. The poor image off the axis is largely due to astigmatism and curvature of field. Coma is also important, but all qualiiied telescope designers are able to eliminate coma.

According to the present invention, a telescopic system is made up in which the objective system is of the type consisting of a Petzval lens and a negative component as a field corrector, and in which the viewing system is broadly of any known type. The field of the telescopic system is overcorrected and matches that of the viewing system, which may include a positive field lens immediately following the field corrector. Altman also employed the overcorrections of field and Petzval sum, but it is surprising that excellent quality can be obtained when this feature is incorporated in a telescope whose objective is a Petzval lens with field corrector. The field corrector of course contributes a large portion of the overcorrection.

I have discovered that this combination gives a remarkably sharp image over the fulli-iieldof View of the viewing system. Moreover, the manufacturing costs are low, due to the :simplefconstruction and the comparatively weakv curvesI of the Petzval type lens. The cost is onlyislightly greater than that of telescopes with r"standard types of objective systems, while the quality compares very favorably with that gained by the complicated objectives disclosed by Altman.

The use of a negative component as a eld corrector with ordinary telescopic systems (not with Petzval lenses) was proposed a number of years ago, but it s'uiers from certain drawbacks, and soon fell into disuse. that' the negative component has a dispersive effect upon the principal rays .of the system. Either this results in very serious loss of illumination at the edges of the field or a positive field lens of considerable power must be used to counteract the dispersive eiect of the negative corrector and the field correcting effect is then largely neutralized also.

The peculiar advantages of the Petzval type lens for use with a negative field corrector in telescope systems seems to have escaped notice until the present time. The zonal spherical aberration of the Petzval lens is known to be remarkably good, as well as its sphere-chromatlsm. It is also said to tend to decrease the secondary spectrum. Petzval lenses without iield correctors have been Its chief disadvantage is used as telescope objectives, but the particular usefulness of such lenses in combinationwith the field corrector lens lies in the fact that the rear positive component is situated comparatively close to the focal plane and has to some degree the effect of a field lens so that a weaker positive field lens suffices behind the focal plane in place of a strong field lens otherwise required, or, in small field viewing systems, the present invention supplies all the field lens effect needed and no addi- -tional element is necessary.

Incidentally, objective systems, according to the invention are particularly adaptable to be used with an entrance pupil at some distance in front, for example in the scanning prism of a periscope. The problem here is largely one of correcting coma, and in the Petzval type lens the coma is easy to adjust by bending the components.

Thus the invention differs from that of Altman referred to above. and from all prior telescopic systems by employing an objective system of the type known as a Petzval lens with field corrector. This lens type is well known in photography where the field corrector is commonly referred to as a field fiattener, and hence the term is a complete description by itself. In a preferred form of the present invention the field corrector is so strong that it overcorrects the field. Hence the use of the term field flattener would be a little inaccurate, and the self-explanatory term "field corrector is used instead.

The Petzval type lens was developed from the original portrait lens of Petzval and is still widely used in photography, with or without the field fiattener. The type may be broadly described as comprisingr two rather widely separated positive components. The optical separation, defined as the distance from the rear principal point of the front component to the front principal point of the rear component, is generally between IAF and 11/2F where F is the focal length of the objective system. In complexity, known lenses of this type vary from those having a. cemented doublet and a simple positive element on the one hand to those having three elements in each component including at least one element of each sign. There is nothing to prevent the use of still more complex forma. if a designer wishes still larger apertures.

Although the invention is fundamentally concerned with the objective system, a few words may be said about the viewing system. It is obvious that the entire viewing system must lie behind the focal plane of the objective system; otherwise the negative field corrector would be in its way. Thus the use of a Huygenian type eyepiece by itself is definitely prevented, but it may be used if a relay lens is interposed. Aside from this limitation a large variety of viewing systems are known, and objective systems according to the invention may be used to great advantage with any of them. A slight modificationv of curvatures may be advantageous to adjust the coma correction for the changed pupil distance. and this can be done in known manner.

*..The point is that different known types of eyepicces are best suited to different angular (apparent) fields of View, and that a skilled optical designer will select a type known to be suitable for the-angular field required by the problem in hand. He will then adapt it to the pupil distance of that part of the optical system which lies in front of the eyepiece, including an objective according to the invention and with or without a relay lens and cooperating positive field lens, and

'4 he will thus be able to reap the benefits of thc invention in that the image will bc sharp throughout a wider angle of field than with previously known systems oi such simple structure.

The type of eyepiece comprising three positivo components, at least one of which is compound, is one of the best for covering an extremely wide field, but of course a less expensive one should be used if it satisfies the demands of use.

In the accompanying drawings:

Figs. 1 and 2 show a telescope according to the invention and constructional data for the same.

Figs. 3 and 4 show similarly another embodiment of the invention.

In Fig. l, I is a scanning prism by which the direction of the line of sight may be changed. An antirotation prism II acts as the entrance pupil of the optical system. The objective system consists of lens elements III to VIII inclusive, VII and VIII forming the field corrector thereof.

The viewing system consists of a positive field lens IX immediately behind the objective system,

`a relay lens of four elements X to XIII, a penta prism XIV, and a highly corrected eyepiece of five elements XV to XIX.

In Fig. 3 the objective system consists of four elements I to IV including a front positive member I, II, a rear positive member III and a. field corrector IV. The viewing system includes a positive field lens V in marginal contact with the field corrector. a relay lens group VI to 1X, and an eyepiece X to XII of a less complex type.

There are certain details of construction which I have found advantageous in applying the invention to specific optical systems, and for the purpose of describing these preferred detailsthe objective system`may be regarded as consisting of three members, a front positive member, a second positive member, and a rear negative member. The two positive members are preferably spaced apart a distance between 0.5F and F measured between their respective facing principal points, where F is the focal length of the objective system. Each of the two positive members preferably has a focal length greater than six-tenths that of the other.

A suitable front member comprises at least two elements including a biconvex element and a negative element of higherrefractive index and lower dispersive index. These may conveniently be cemented together. The front bounding surface of the front member may advantageously be given a dioptric power between 0.35 and 1.0 times that of the entire front member.

The second positive member may advantageously have a convex front surface and a weakly curved rear surface with a radius of curvature greater than 1.2F. The rear negative member is in front of and very close to the focal plane, preferably less than 0.08F therefrom and its rear bounding surface should be weaker than its front bounding surface. Obviously the front bounding surface would then necessarily be concave. Good results have been obtained by making the rear surface with dioptric power algebraically between +0.? and -0.2 times that of the front surface. According to one feature of the invention. the rear surface may be given a small or zero curvature approximating that of the image surface and may be suitably marked for use as a reticle.

The data for the examples shown in the drawings are repeated here:

Emploi. Figs. 1 and 2 annoso [Magnieatlon-BX] 1m N V Badil Thicknelees I. Right Angle Prism. 1.517 64.5 40x40 mm. h-grrm.

lill II. Do'olrllm 1.517 64.5 35x25 Isl-25 n- IIL 1.572 l57.4 Rin-H0408 0,5

l l- 1V.... --...f:.- 1.617 66.6 Ren-37% l lg-3.5

Rw-123.02 esl-83.7 V 1.617 55.0 Rin-H482 los r- VI 1.617 36.6 Rel-1NR h 45 Remi-145.19

' H185 VII 1.517 64.5 Run-41.16 t o 1- VIII. 1.564 46.0 .Rl-+4532 n 3 man. 12.--

n-4.0 IX 1.075 32.2 R10-70.03

.Ru-32.77 l '1I-115.2

x 1.010 aas leu-Hoezo hel-2.9

XI 1.523 56.6 Rin-+4061 til-4.6

Riel-105.03 l "-4.0 XII 1.523 56.6 Rie-+5333 t 46 u- XIII 1.649 $4.8 RMI-64.31

tui-2.9 R11-l 81=4-0 XIV; PentagonPrism. 1.517 64.5 35x36 tir-g2?) Run-107.40 tui-5.9 XV 1.673 32.2 Rie-+5130' liein- XVI 1.517 64.5 Raon-51.30

Ixo-1.2 Rnl--i-lwl' |112.7 XVII 1.611 58.8 Raw-130m liv-1.2 Ris-+4209 1 Cul-18.2 XVIII 1.517 64.5 Ru54.45 t 59 nxix 1.073 sa: 1ra-neem Example 2, Figs. 3 and 4 [Magnification-3X] Lens ND V Radli Thicknesses I 1.617 36.6 Riehl-6.5.x t 26 1- II 1.572 57.4 R|-+21.46

la-S lei-lease ll-47.0 III 1.517 64.5 R-+74.60

y lai-3.3 Ri-o .1-273 IV 1.525 54.8 Iig-46.75

:gl-1.3 V 1.584 46.0 Rr-m 11-7.1 Rel-12.51

tel-28.8 VI 1.621 36.2 Rui-+3101 t 14 6* VII 1.523 56.6 liu-+1038' t1-5.4 RWI-61.62 l1='23.6 VIII 1.621 36.2 R11-+2270 t 18 lim sae 12u-+1119 h-LB Ru--N4A0 "-98.6 1.649. 33.6 Riel-1&1?

11g-5.o 1. 511 64. 5 R11-+4816 tlv-21.2 Rill-48.78 "-3.6 1. 517 64. 5 R1e"+81.62 v Iig-14.3

Raw-81.62

6 A number of notable features of these two telescopes are summarized in the following table, including the preferred features of the invention not directly evident from the above table of data:

,EL 1 Ex. 2

Focal length of objective system:

(l) as a whole (F) 100.0 mm 80.5

2 front member alone.. +1.50F +1.17F (3) second member alone +0.00? +1.81* (4) negative member alone 0.01F 0.25F

Two front members: I

(5) Ratio of focal length (3):(2).. 0.66 1.54 (6) Spacing apart 0.8413' 0.58F

Front member alone:

Index dillerence, Nix-Nr +.045 .045 8) Dioptric power oi whole front member +6.67 +10.8 (9) Dioptric power of front surface one.. +2.95 +9.4

(l0) Ratio (9):(8) +0.44 +0.81

Second member alone:

(l1) Radius oi curvature, iront sur ace +0.45F +0.93F (12) Radius of curvature, rear surface +1.45F u .Negative member alone:

Distance from focal plane..." 0.00 0.0101T 14 Dioptricpower offrent surface. -12.5 31.37 15) Dicptric power of rear surface. 0.0 -13.57 (1B) Rat o (15):(14) 0.0 +0.43

Field telngle at which field was com- 10 3.5

u p (17) sagittal field +1.20 mm +1.46 mm (18) T angentiai field +5.78 mm 2.11 mm Magnification ratio of erecting system. 1.17 2.15 Focal length of eyepiece 39.1 57.1

From these tables and the figures it is readily seen that in both examples the two front members are spaced apart a' distance between 0.51" and F and each of them has a focal length greater than 0.6 times that ofthe other. However, in Ex. 1 the second component has the shorter focal length, and in Ex. 2 it has the longer.

The front member consists entirely of a. single component made up of a positive element ce mented to a negative element. In Ex. 1 the positiev element is in front Whereas in Ex. 2 it is behind the lnegative element. In each case the refractive index of the negative element is greater than that of the positive element by 0.045, As is well known to lens designers, this diierence in index should be greater than 0.01. 0n the other hand, I 11nd it preferable that it be less than about 0.15 or 0.20.

The second member consists of a single component in both examples. In Ex, 1 itis a ce mented doublet purely for correcting chromatic aberrations, the refractive indices of the two elements being as nearly the same as practicable. Making this index greater than 1.6 is helpful in controlling the Petzval sum. In Ex. 2 this component is a. simple positive element plano-convex and very economical to manufacture.

It is to be pointed out that within the scope ofthe invention either of the front -two members may be made of a more complex structure for the purpose of further correcting the aberrations so as to use a larger aperture or field.

The front surface of the -second member is convex in b oth examples. The rear surface is concave in one example and plano in the other, the radius of curvature being greater than 1.2 F in each case. It could also be weakly convex, but if it is convex the radius of curvature is prefer ably greater than 2.0 F.

In the negative member close to the focal plane. the dioptric power of the rear surface is algebraically between +0.? (which would mean a concave rear surface) and 0.2 (which would mean a convex rear surface) times that of the front surface of the negative member.

Although the present invention is primarily directed toward improvements in the objective system, there are also certain features of the viewing system which cooperate therewith, In order to use the invention to the fullest advantage, the viewing system must have large enough apertures substantially to match those of the objective as regards both pupil size and angular field. Some types of eyepiece are superior in covering a wide field, particularly the known type comprising three positive components of which at least one is compound. Example 1 shows this type of eyepiece. Example 2 was designed for less exacting requirements. i. e, for a small angular field, and since the problem was to improve an existing telescope with few changes. the same eyepiece was retained without change. It is. of course. more economical to manufacture than the eyepiece of Example 1.

Both of these telescopes are of a commonly used type in which the viewing system comprises a field lens, an erecting or relay lens. an eyepiece and, optionally, prisms and other accessories. In applying the invention to this type of telescope. I have found it preferable to mount the field lens less than 0.1F from the rear surface oi the objective. In Example 2 the field lens is actually in peripheral contact with the rear of the objective. The front surface of the field lens is preferably weaker than the rear surface, I have found as in the case of the negative member of the objective, that this surface may conveniently be made with its dioptric power algebraically between +0.7 and 0.2 times that of the other surface of the field lens, I'he refractive index of this lens should be above 1.56 and preferably above 1.64, and may be as high as suitable glasses permit. Glasses are known with indices about 2.1.

It is usual in telescopes of this type for the relay lens to work at a magnification between 0.5 and 4.0.

These examples embody some of the principles disclosed in Altmans copending application Serial No. 511,062, Sighting devices, in that the curvature of field of the objective system is overcorrected and substantially matches the undercorrected curvature of field of the viewing system. Furthermore, in Example 1 the objective has eight optical surfaces not counting the one coinciding with the focal plane and therefore having little or no effect on the aberration. This fact further corroborates Altmanrs statement, in the case above referred to, that at least eight refracting surfaces'are essential in an' objective forga sighting system covering a. wide angle. .Example 2 has less than eight. but it covers a more mod. erate angle of iield.

What I claim is:

1. A sighting telescope comprising in combination an objective system for forming substantially at its principal focal plane an image of the object being sighted and a viewing system for viewing said image. all in optical alignment, in which the objective system comprises three members namely a frontpositive member, a second positive'member and a. negative member close to and in front of the focal plane, the two positive members be'- ing spaced apart between 0.511' and F where F is the focal length of the objective. each of the two positive members having a focal length greater than 0.8 times that of the other, the front member comprising at least two elements including a biconvex element cemented to a nega-l tive element of higher refractive index and lower dispersive index, the front bounding surface of the front member having a dioptric power between 0.35 and 1.0 times that of the entire front member, the second positive member having a convex front surface and a weakly curved rear surface with a radius of curvature greater than 1.2F, the negative member consisting of an unsymmetrical component with its weaker surface toward the rear. and the viewing system being entirely behind the focal plane of the objective and including a compound eyepiece consisting of a plurality of components at least one or which is compound. and' which are spaced apart an aggregate distance less than one-third the focal length of the eyepiece.

2. A sighting telescope according to claim 1 in which each positive member of the objective consists of at most two elements.

3. A sighting telescope according to claim 1 in which the viewing system includes as one of its members an unsymmetrical positive component with its weaker surface to the front adjacent the rear of the objective system and spaced less than 0.1F therefrom.

4. A sighting telescope according to claim 1 in which both positive members of the objective system are compound, each consisting of a biconvex element cemented to a negative velement of lower dispersive index.

5. A sighting telescope in which the objective system is substantially according to the following specifications:

where the first column designates the lens elements by Roman numerals in order from front to rear and when-.Fis the focal length of the objective, N is the index of refraction for the D line of the spectrum, Vis the dispersive index, R. t, and s refer respectively to 'the radii of curvature of the refractive surfaces, the axial thicknesses of the elements, and the airspaces between the components, the subscripts on these refer to the spaces numbered consecutively from the front and the and signs in the fourth column correspond to surfaces which are respectively convex and concave to the front.

6. A sighting telescope in which the objective 9 system is substantially according to the following specincations:

where the first column designates the lens elements by Roman numerals in order from front to rear'and where F is the focal length of the objective.- N is the index of refraction for the D line of the spectrum, V is the dispersive index, R, t, and s refer respectively to the radii of curvature of the refractive surfaces, the axial thick,- nesses of the elements,l and the airspaces between the components, the subscripts on these refer to the surfaces.. the elements and the spaces numbered consecutively from the front, and the and signs in the fourth column correspond to surfaces which are respectively convex and` concave to the front. a

7. A telescopic system comprising an objective system and a viewing system in optical alignment on opposite sides of a focal plane, in which the Yobjective system comprises'two positive members each with a dioptrie power greater than 0.6 times that of the other and separated a distance between principal points greater than 0.2511* and 10 less than 1.511' where F is the focal length of the objectlvesystem, and at least the front lone of 'the positive members being compound, and in combination therewith a single negative compo- 5 nent as a. eld correcting lens immediately adiav cent to the focal plane, the objective system forming a backward curving image of distant objects at the focal plane, and in which the viewing system has a iield curvature substantially matching that of the objective system and comprises at least three positive components of which at least one is compound and another is a simple positive element with its weaker surface facing the focal plane and separated therefrom by less than 0.111.

WILLY SCHADE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 815,657 Swasey Mar. 20, 1906 869,395 Schleth Oct. 29, 1907 940,894 vVon Rohr Nov. 23, 1909 1,468,762 Taylor et al. Sept. 25, 1923 1,478,704 Erile Dec. 25, 1923 .1,865,977 Sonnefeld July 5, 1932 2,050,024 Sonnefeld Aug. 4, 1936 2,076,190 Wood Apr. 6, 1937 2,187,780 Gehrke et al. Jan. 23, 1940 2,276,284 Burka et al. Mar. 17, 1942 FOREIGN PATENTS 86 Number Country Date 501,456 Germany July 14, 1930 

